The purpose of the proposed study will be to expand our knowledge of the location and shape of respirator faceseal leak sites and their association with critical facial dimensions of test subjects. This will be accomplished through the application of equipment and methodologies developed during our previous study to identify faceseal leak sites on subjects wearing a half-mask respirator. The specific objectives of the proposed study are to: 1) determine if leak sites and shapes change between the exercises of a quantitative fit test, 2) determine the variation of leak sites and shapes between multiple fittings of the s respirator on the same subject, 3) determine the variation of leak sites and shapes between different respirators being worn by the same subjects, 4) more accurately define the facial dimensions of subjects who show evidence of aerodynamic streamlining beneath the respirator, and 5) determine the effect of increased breathing rates on the development of air flow streamlining patterns. The first four determinations will be made by performing respirator fit tests on human subjects using an aerosol of a fluorescent whitening agent. Faceseal leak sites will be identified by observation of fluorescence of the aerosol sites at the site(s) when illuminated by ultraviolet light. The fifth determination will be by performing the same tests on a mannequin fitted with a respirator using a breathing simulator. Completion of this work will be preceded by selection of alternative tracer materials based on toxicological and fluorescent properties, modification of the aerosol generating, conditioning and exposure system developed under our previous study, and characterization of alternative tracer aerosols.